Iván Cámara Mayorga

Ultra-wide-band traveling-wave photodetectors for photonic local oscillators

This paper reviews recent advances in the development of high-speed 1.55-/spl mu/m traveling-wave p-i-n photodetectors (TWPD) for photonic millimeter-wave and submillimeter-wave local oscillators. We first discuss the basic physics and performances of high-speed 1.55-/spl mu/m TWPD. Next, we present a frequency-domain optical-heterodyne measurement technique for ultra-wide-band characterization of the TWPD and photonic transmitter modules within the frequency range from almost dc up to more than 1 THz. We further demonstrate ultra-wide-band (0.02-0.7 THz) photonic transmitter modules consisting of a high-speed TWPD coupled to a broad-band bow-tie antenna as well as a narrow-band 0.46-THz photonic transmitter module producing output power levels sufficient to operate a superconductor-insulator-superconductor (SIS) astronomical receiver under optimum conditions. Finally, we will report on ultra-wide-band (0.06-1 THz) photonic transmitter modules consisting of high-speed TWPDs coupled to various rectangular metallic waveguides (WR10, WR8, and WR5).

Continuous-wave optical parametric terahertz source

Here, we present a continuous-wave optical parametric terahertz light source that does not require cooling. It coherently emits a diffraction-limited terahertz beam that is tunable from 1.3 to 1.7 THz with power levels exceeding 1 microW. Simultaneous phase matching of two nonlinear processes within one periodically-poled lithium niobate crystal, situated in an optical resonator, is employed: The signal wave of a primary parametric process is enhanced in this resonator. Therefore, its power is sufficient for starting a second process, generating a backwards traveling terahertz wave. Such a scheme of cascaded processes increases the output power of a terahertz system by more than one order of magnitude compared with non-resonant difference frequency generation due to high intracavity powers. The existence of linearly polarized terahertz radiation at 1.35 THz is confirmed by analyzing the terahertz light with metal grid polarizers and a Fabry-Pérot interferometer.

New Two-Color Laser Concepts for THz Generation

Two-color semiconductor external cavity laser concepts for terahertz (THz) generation are discussed. By defining three critical characteristics, various two-color laser configurations are experimentally classified with respect to the application of THz generation. According to our experimental results, we suggest a new two-color laser configuration. It is based on an external cavity with an etalon.

First In-Field Application of a Full Photonic Local Oscillator to Terahertz Astronomy

This letter reports on our recent improvements in photomixing technology for the realization of a photonic local oscillator (LO) at 1.05 THz for the Atacama Pathfinder Experiment (APEX) radio telescope. Experiments with state-of-the-art photomixers, operated at room temperature and in cryogenic environment demonstrate successful operation of an astronomical heterodyne receiver at 1050 GHz with a superconductor-insulator-superconductor (SIS) mixer. The system noise temperature of the heterodyne receiver pumped by the photonic LO was at least as low as that using a conventional solid-state LO in the same receiver system. An optical comb generator served as a relative frequency reference to which both lasers were phase-locked. Under the phase lock condition, the 3 dB linewidth of the THz signal was below 3 kHz and could be continuously tuned within a range of 500 MHz-the overall tunability of the system was determined by the photomixer antenna resonance bandwidth, which was roughly 200 GHz. We installed the laser system in the telescope pedestal, from there, the frequency-stabilized laser signal, was fed into the photomixer, installed in the Nasmyth cabin of the telescope, through a 20 meters long single-mode fiber optic.

Compact diode-laser-based system for continuous-wave and quasi-time-domain terahertz spectroscopy

We present a multimodal diode-laser-based terahertz (THz) spectroscopy system. In contrast to other laser-based THz setups that provide either cw or broadband THz generation, our configuration combines the advantages of both approaches. Our low complexity setup enables fast switching from cw difference frequency generation to broadband THz emission, enabling sophisticated data analysis like much more complex time domain spectroscopy systems.

Generation of continuous-wave terahertz radiation using a two-mode titanium sapphire laser containing an intracavity Fabry–Perot etalon

Continuous-wave terahertz (THz) radiation was generated by photomixing two modes of a titanium sapphire laser. The laser was induced to oscillate on two modes by placing a Fabry–Perot etalon in the laser resonator. The frequency of terahertz radiation, which was equal to the difference frequency of the two modes, was varied by adjusting the free spectral range (FSR) of the etalon. Photomixing was performed by logarithmic spiral antennas fabricated on low-temperature-grown GaAs; and the emitted THz radiation was characterized. The THz power, measured by a Golay cell, was 1μW at 0.3THz and 0.7μW at 0.5THz. The THz frequency, as determined by a Fourier transform interferometer, was seen to correspond to the etalon FSR. The current-voltage characteristics of photomixers were also determined, and photocurrent modulation was observed by the autocorrelation of the laser beam.

An Optimization of Terahertz Local Oscillators based on LT-GaAs Technology.

We report on photonic technologies developed at the MPI fur Radioastronomy and the Research Center Julich to generate Terahertz Local Oscillator reference signals for use on e.g. ALMA/APEX and SOFIA. The principle is to mix two (NIR) laser colours in a biased LTG-GaAs layer, thus creating a high-frequency beat (difference) frequency signal. This output signal is coupled to free space through an antenna. In this work a systematic study of the photomixer design, in order to optimize the RF power, is presented. Part of the experiments were done with photomixers integrated to with a broadband spiral antenna designed for frequencies up to 1 THz. The LT GaAs photomixers are prepared on materials with various growth temperatures as well as using resonant cavity material structures and various finger contact geometries. An improvement in the output power up to around 3 μW of submillimeter radiation (0.5 THz) is demonstrated.

Enhancing the stability of a continuous-wave terahertz system by photocurrent normalization

In a continuous-wave terahertz system based on photomixing, the measured amplitude of the terahertz signal shows a variability due to drifts of the responsivities of the photomixers and of the optical power illuminating the photomixers. We report a simple method to substantially reduce this variability. By normalizing the amplitude to the DC photocurrents in both the transmitter and receiver photomixers, we achieve a significant increase in stability. If, e.g., the optical power of one laser is reduced by 10%, the normalized signal is expected to change by only 0.3%, i.e., less than the typical uncertainty due to short-term fluctuations. This stabilization can be particularly valuable for terahertz applications in nonideal environmental conditions outside of a temperature-stabilized laboratory.

Note: Coherent detection of terahertz radiation employing a continuous wave optical parametric source.

The combination of an all-optical terahertz source with a photoconductive antenna to achieve coherent detection is presented. This approach aims to overcome the frequency limits introduced by optoelectronic terahertz sources commonly used. Here the Gaussian-shaped and linearly polarized terahertz waves are generated by a continuous wave optical parametric oscillator with a power of 3 μW at 1.4 THz. The infrared signal light of the optical parametric oscillator can be used to coherently detect the generated terahertz wave with a photoconductive antenna. As a proof-of-principle experiment we determine the thickness profile of a plastic lens using phase shifting interferometry.

Cw terahertz spectrometer with high-precision frequency control

We realized a continuous-wave terahertz spectrometer based on optical heterodyning of two near-infrared distributed-feedback diode lasers. Using active frequency stabilization we achieve 1 MHz resolution and a signal-to-noise ratio up to 80 dB.